A hydraulic power steering gear for steering a vehicle includes a directional control valve assembly. The steering gear also includes a piston and a mechanical steering linkage which is connected between the piston and the steerable wheels of the vehicle. The directional control valve assembly operates in response to rotation of an input shaft which is connected to the steering wheel of the vehicle. Depending upon the direction in which the steering wheel and the input shaft are turned by the operator of the vehicle, the directional control valve assembly directs a pressurized flow of hydraulic fluid to either one of two sides of the piston. The directional control valve assembly simultaneously directs the hydraulic fluid from the other side of the piston to the fluid reservoir. The directional control valve assembly thus pressurizes one side of the piston and vents the other side of the piston. The hydraulic fluid pressure acting on the piston then causes the piston to move in the selected direction. The mechanical steering linkage responds to movement of the piston by turning the steerable wheels in the selected direction.
The directional control valve assembly includes a valve core and a valve sleeve. The valve core and the valve sleeve have surfaces which define hydraulic fluid flow paths through the valve assembly, and are supported to rotate relative to each other. The valve core is connected to the input shaft to rotate relative to the valve sleeve when the operator of the vehicle turns the steering wheel. The valve sleeve is connected to the piston to rotate relative to the valve core when the piston moves. When the valve core is rotated relative to the valve sleeve upon rotation of the steering wheel, the hydraulic fluid flow paths defined by the valve core and the valve sleeve are adjusted so as to pressurize one side of the piston and simultaneously to vent the other side of the piston. The piston then moves and the steerable vehicle wheels are steered accordingly. When the piston is thus moved to effect steering of the vehicle wheels, the valve sleeve is rotated relative to the valve core in a follow-up manner. The valve sleeve and the valve core are thus returned to their centered positions as steering of the wheels in the selected direction is completed.
The pressurized flows of hydraulic fluid in the directional control valve assembly cause the valve core to vibrate relative to the valve sleeve. The vibrations can cause noises that are audible to the occupants of the vehicle.